The former colonial power sent fighter jets and troops two months ago to eliminate al Qaeda-linked groups controlling the north of the country and threatening to move south towards the capital.

The jihadist network in Mali has funded itself by taking foreign hostages, but also levying a tax on smugglers running drugs from Latin America to Europe.

Poverty and the lack of government presence in the vast desert has provided an ideal ground for smugglers.

Typically, the drugs are shipped to the Gulf of Guinea or flown in directly from countries including Venezuela into Mauritania or Mali, where they are stored and eventually taken overland to the Mediterranean's southern shores.

The route is known as "Highway 10" in reference to the 10th parallel, a line of latitude which cuts through Colombia and Venezuela at one end, Guinea and Nigeria at the other and just misses Mali.

In a report, the UN Office on Drugs and Crime said that around 10% of the 172 tons of pure cocaine that entered Europe in 2010 transited through West Africa.

French researcher Mathieu Guidere said the military intervention in Mali had "totally disrupted the trafficking of drugs, weapons and migrants in the region, smashing up all the networks transiting through northern Mali".

Mr Guidere said smugglers have been paying a fee worth around 10% of their cargo's value to groups such as al Qaeda in the Islamic Maghreb.

He added: "Some groups would even, for an additional fee, offer protection for the convoys."

But smugglers are already finding new routes through Angola, the Republic of Congo, the Great Lakes region in eastern Africa and post-war Libya.

Criminologist Xavier Raufer said: "Profit margins in the cocaine trafficking business are so huge that longer smuggling routes and subsequently higher transport costs are not a problem."

France has said a large part of its task in Mali is already done and has vowed to start scaling back its deployment next month.